My goal is to eliminate all AA batteries in my project. Can I use one specific DC power supply to power five separate circuits, all currently powered by AA batteries? One of the circuits requires 8V, two of them are at 6V each, one is at 4.5V, and the last one is at 3V. Please let me know if it can be done using existing products, or if I will need to create it using regulators and voltage dividers, etc. Please let me know what voltage DC power-supply I will need for this project.
The simplest solution may be to just buy a 9V or 12V AC power adapter.
You didn't mention the currents in your circuits. Without that it's pretty difficult to find an appropriate solution.
You didn't mention if you preferred through-hole components or SMD ones. If you can, I'd say go for SMD for currents < 0.2A because those are much cheaper. If you can't get something like TO-92 for 0.2A or TO-220 for more.
If you work with low currents (low powers) and don't care too much about efficiency you can then obtain lower voltages by using some LDO regulators. You can search the common distributor sites (Mouser, Digikey, Farnell, or more hobbyst sites like Futurlec or maybe Sparkfun) for those:
The 4.5V LDO is less common. You can therefore either:
Another possible solution may be to use the integrated DC/DC converters. These provide much higher efficiency, though they're (a lot) more expensive. In my opinion for low currents their cost is not justified unless you'd be running on (small) batteries which doesn't seem to be the case.
As for voltage dividers (using resistors) it may not be such a bad idea, expecially for low currents and if your circuits aren't very sensitive to their supply voltage (e.g. they can accept 4.2V instead of 4.5V for instance). It may be the cheapest solution of all though it has a few drawbacks:
I'll post some schematics later if you need them (please let me know).
You can get DC-DC Buck converters for very few dollars US. (Sorry if that is what user51166 meant by "integrated DC-DC converters.) For example Example Buck Converter. You'll probably want 6, but could power all from a common supply. Since your DC-DC buck converters are going to only be about 90% efficient your total Voltage needed to run those 6 is about 33 Volts (and you'll want a little extra on your common supply to make up for the inefficiency of the Buck converters. A 36 Volt switching power supply is going to cost close to $40 USD or more.
You're replacing about 19 AA Batteries. Your cost is about $5.90 USD in bulk so your payback is about $50/$5.90 = 8.4 changes of batteries. Might be worth it if you run those devices often. But the items are going to need to be fairly near the common switch mode supply or your going to have a spider's web of wires running around.